JIM LEHRER: Now, the surprising connection between family, friends and obesity. That idea was outlined today in a study published in the New England Journal of Medicine. Researchers found that a risk of obesity seems to significantly rise through personal relationships.

JIM LEHRER: Take us through this. How would an association with family and friends cause you to be obese?

SUSAN DENTZER: Well, Jim, the researchers who did this study, Dr. Nicholas Christakis from Harvard, who’s a medical sociologist, and James Fowler, his colleague, who’s a political scientist from the University of California, San Diego, looked at the fact that we have, as we know, an obesity epidemic. And they asked, “Gee, could this be spreading the way a flu epidemic spreads?”

And so what they did is they went to a very valuable source of data, which is the long-term Framingham Heart Study. That’s the study that, over many years, has looked at risks of heart disease, originally based in Framingham, Massachusetts, and evaluated people over the years to see what happens, what happens when they gain weight, et cetera, et cetera.

An interesting facet of that study is that people were all asked, “Who are your friends? Who are your relatives? Who are your siblings?” Partly just so the researchers can keep in touch with people over years, so they had all these records of the people in the study. And it turns out that, when they name their friends or relatives, because it was in Framingham a lot of those people were in the study, too.

So what you ended up was this large group of people in the study, in this study 12,000 people, who were all friends or relatives of each other, and they were able to track what happened as these people were assessed, particularly for their weight, over a period from 1971 to 2003. They took that data; they built a statistical model on a computer and basically found that, looking across these various associations, you see these connections.

For example, if your spouse is obese, the risk that you will become obese increases by 37 percent. If your sibling is obese, becomes obese over time, the risk that you will become obese increases by 40 percent. If your friend becomes obese, the risk goes up by 57 percent. And if both of you are kind of close friends, you basically both name each other as your friend, the risk can rise as much as 171 percent.

Impact of social influence

JIM LEHRER: A hundred and seventy-one percent? Now, the siblings and members of the family could possibly be explained by genetics or whatever, but friends, no, right?

SUSAN DENTZER: Well, and they actually controlled -- they took out of the data all of those relationships they were already aware of -- the genetic risk, for example, as you say, which is pronounced. They also controlled for other things that we know, the so-called phenomenon, "birds of a feather flock together." If you're kind of heavy, you tend to hang out with heavier people. Also, you tend to...

JIM LEHRER: That's the social influence, right?

SUSAN DENTZER: That's another influence. But they also were able to control for that, so they basically took out all these things that we know about that are associations, and they said, "What's left?" And what's left was this big effect, which they attribute to what we call social norms. It's basically a sense that, "Well, if I see my friends getting obese, it looks like that's OK to me, and so I am more inclined to become obese, as well."

JIM LEHRER: But it takes, in the first instance, there must be obesity, right?

SUSAN DENTZER: Right.

JIM LEHRER: And then that person can then influence somebody else by just being with them, just having some kind of social relationship with them?

SUSAN DENTZER: Yes, and the social relationship is the important piece, because they found that these patterns held even if friends lived 500 miles apart, you know, they only saw each other once a year at Thanksgiving. The fact that you move in a social network, even if it's distributed broadly across geography, means that you're influenced. "We live in a network," they wrote in the study, "and our health is impacted by the fact that we live in a network."

Thin people have thin friends

JIM LEHRER: Was the reverse true, that thin people tend to also have thin friends and thin members of their family?

SUSAN DENTZER: Yes, or at least by inference, that's right, by inference. And the big point of all of this, of course, other than telling us that we all live in social networks, we're embedded in social networks, and we tend to pass around these behavioral norms and all share them and replicate them, what the implications of this are really great for is how you would attack obesity.

Because what it tells you is, if you could persuade one person in a network to eat less, exercise more, lose weight, you're likely to influence all of the people, at least a lot of the people in that person's strong social network. So when you think in terms of interventions of some sort, you know, maybe you spend $1,000 dollars to get somebody a health club membership, it's not going to just reap benefits for that person. The benefits are going to spread potentially throughout the social network.

JIM LEHRER: But if somebody is obese and does something about it, like going to a health club, other kinds of exercise, diet and all of that, these researchers would argue that that will cause others who are also -- who started to be -- who were also obese, to do the same kinds of good things?

SUSAN DENTZER: Yes, that norm, the norm of being thin, will be as contagious as the norm of being fat; that's at least the hypothesis.

Serious health risks of obesity

JIM LEHRER: Remind us just for the record -- I know you've mentioned this on this program many times, and all health experts know this, just what a serious health risk obesity has become in this country right now.

SUSAN DENTZER: Well, we know that the number of U.S. adults, the proportion of U.S. adults who are overweight or obese -- overweight is now two-thirds of the U.S. adult population. And we also know that there are at least 9 million obese kids and that the rate of obesity among children has more than doubled or tripled over the last three years.

We also know that obesity is linked to all kinds of deleterious health outcomes, from diabetes to hypertension, and increasingly we know it's also linked to cancer. So the unfortunate thing is, as these social norms of obesity spread, these diseases, in fact, are in a way kind of spreading right along with them.

JIM LEHRER: And they can be tracked. In other words, as obesity rates go up, diabetes rates go up, for instance, and almost in a direct way?

SUSAN DENTZER: Absolutely. We see this very much so, and particularly now among kids, where kids are now increasingly developing this Type II diabetes that we used to call adult-onset diabetes but now we see it in kids.

JIM LEHRER: Some experts say, as you know, that this is equivalent to smoking in terms of a health risk in our country today, as a serious matter.

SUSAN DENTZER: And possibly even greater over the long term. It really could reverse a trend that we've had to declining rates of disability in old age, and basically produce more disabled people than we've ever seen.

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